Development of a Library of Novel Compounds for Use in Organic Solar Cells

Abstract

Currently, the most common method of optimising organic solar cells is to synthesise new materials before fabricating and testing devices under a wide range of processing conditions to empirically determine the optimum conditions, with other measurements supporting the explanation. This is both a laborious and material intensive process. Simplification of device optimisation processes would likely enable devices to be more easily manufactured at a lower cost. In turn, it would be expected for organic solar cells to become more accessible to the consumer, decreasing the need for fossil fuels. Two approaches to simplifying organic solar cell manufacture were investigated.First, a series of triphenylamine (TPA) centred materials were synthesised to act as non-fullerene acceptors. However, as all four synthesised TPA materials showed comparable opto-electronic properties which were complimentary to that of common donor P3HT, no distinction towards the relative device performance could be made based on these parameters alone. As bulk heterojunction (BHJ) morphology is critical to device performance, differential scanning calorimetry (DSC) in conjunction with X-ray diffraction (XRD) provided improved screening capabilities. When blended with P3HT at varying concentrations and deposited from solution, the morphological differences of the resulting blended films were probed by thermal methods to successfully predict the optimum donor:acceptor pair. Application of Flory-Huggins theory and Kyu plots to determine the interaction parameter ) reveal that the materials must be miscible to enable low-energy interfaces, while phase separation into pure domains enables charges to be extracted from the device through continuous percolation pathways. The donor:acceptor pair with the highest positive value of demonstrated a PCE of 2.0% and JSC above 4 mA/cm2, while the pair with the lowest value lead to poor device performance (<0.4% PCE) due to excessive mixing. DSC methods were also able to provide a calculated critical solvation concentration (wcrit); below which the donor and acceptor were expected to become immiscible and perform poorly due to limited phase separation and non-ordered regimes. Additional insights provided by DSC analysis include the observation of a cold-crystallisation process; where liquidation upon heating nucleates crystallisation. Typically due to the entrapment of disordered phases, the DSC experiment was able to provide insight towards highly variable device performance, which otherwise may have been attributed to manufacturing errors.Secondly, further simplification of organic solar cells to homojunction (i.e., single component) devices was completed. Materials with low exciton binding energies are able to efficiently separate into free charge carriers and limit recombination pathways. The binding energy of an exciton is dependent on a number of factors, such as that described by the Wannier-Mott construct, which states that the exciton binding energy is proportional to the inverse square of the dielectric constant. Additionally, according to the Clausius-Mossotti relation, the dielectric constant of a material can be related to its density, with increased density providing a higher dielectric constant. As a result, it is expected that increasing the dielectric constant of a material will decrease exciton binding energy, reducing the energy losses for exciton separation. Inclusion of ethylene glycol ether chains are known to dramatically increase the dielectric constant at low frequency, while maintaining both the optical and electronic properties of their alkylated derivatives. Additionally, inclusion of these glycolated units has the ability to increase the molecular packing density of the film, enhancing the high (optical) frequency dielectric constant, and thus, homojunction device performance. Three glycolated materials were synthesised which contained a cyclopentadithiophene core attached to benzothiadiazole units, of which, two were functionalised with aldehyde or vinyl dibromide groups. At low frequencies, dielectric constants up to ~5 were obtained, however it was demonstrated that the dielectric constant at optical frequencies (which has been largely unreported at this time) is most influential towards homojunction performance. The device with the highest optical-frequency constant (~4), showed an EQE response extending to its optical onset; suggestive of decreased exciton binding energy which is also lower than the energy of excitation.In conclusion, the study has provided important insights into the processes required to streamline organic solar cell manufacture. Predictions towards the ideal donor:acceptor pair by DSC methods can minimise time, material and labour costs, while simplifications towards homojunction devices can be successfully achieved by increasing the optical frequency dielectric constant of organic materials.